U.S. patent application number 10/860060 was filed with the patent office on 2005-02-17 for method for forming a si film, si film and solar battery.
This patent application is currently assigned to TOHOKU UNIVERSITY. Invention is credited to Fujiwara, Kozo, Nakajima, Kazuo, Ujihara, Toru, Usami, Noritaka.
Application Number | 20050034756 10/860060 |
Document ID | / |
Family ID | 33296867 |
Filed Date | 2005-02-17 |
United States Patent
Application |
20050034756 |
Kind Code |
A1 |
Nakajima, Kazuo ; et
al. |
February 17, 2005 |
Method for forming a Si film, Si film and solar battery
Abstract
A Si melt is contacted to a main surface of a Si substrate made
of metallurgical Si raw material to conduct liquid phase epitaxy
within a temperature range around Si melting point and to form a Si
crystal thin film on the main surface of the Si substrate.
Inventors: |
Nakajima, Kazuo;
(Taiwamachi, JP) ; Usami, Noritaka; (Sendai City,
JP) ; Ujihara, Toru; (Tagajyo City, JP) ;
Fujiwara, Kozo; (Sendai City, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
TOHOKU UNIVERSITY
Sendai City
JP
|
Family ID: |
33296867 |
Appl. No.: |
10/860060 |
Filed: |
June 4, 2004 |
Current U.S.
Class: |
136/261 ;
257/E21.115; 438/97 |
Current CPC
Class: |
H01L 21/02532 20130101;
Y02P 70/521 20151101; C30B 29/06 20130101; Y02P 70/50 20151101;
H01L 21/02573 20130101; Y02E 10/547 20130101; H01L 31/1804
20130101; H01L 21/02625 20130101; C30B 19/00 20130101; H01L
21/02381 20130101 |
Class at
Publication: |
136/261 ;
438/097 |
International
Class: |
H01L 031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2003 |
JP |
2003-167493 |
Claims
What is claimed is:
1. A method for forming a Si thin film, comprising the steps of:
preparing a Si melt kept at a temperature near Si melting point,
preparing a Si substrate made of Si single crystal or Si
polycrystal, and contacting said Si melt to a main surface of said
Si substrate to conduct liquid phase epitaxy within a temperature
range around said Si melting point and to form a Si crystal thin
film on said main surface of said Si substrate.
2. The forming method as defined in claim 1, wherein said Si
substrate is made of metallurgical Si raw material.
3. The forming method as defined in claim 1, wherein said liquid
phase epitaxy is conducted within a temperature range of
.+-.5.degree. C. of said Si melting point.
4. The forming method as defined in claim 1, wherein said liquid
phase epitaxy is conducted under the condition that said Si melt is
maintained within a temperature range between said Si melting point
and .+-.5.degree. C. of said Si melting point, and a portion of
said Si substrate is contacted to said Si melt, to be melted, and a
temperature of a region of said Si melt in the vicinity of said Si
substrate is maintained lower than said Si melting point.
5. The forming method as defined in claim 4, wherein said liquid
phase epitaxy includes the steps of: supporting said Si melt in a
melt holder formed at a sliding type first member, supporting said
Si substrate in a substrate holder formed at a sliding type second
member, sliding at least one of said first member and said second
member when a temperature of said Si melt approaches to a first
temperature around said Si melting point, to contact said Si melt
to said main surface of said Si substrate and to initiate growth of
said Si crystal thin film on said main surface of said Si substrate
through epitaxial growth, and sliding at least one of said first
member and said second member when said temperature of said Si melt
approaches to a second temperature lower than the first
temperature, to leave away said Si melt from said main surface of
said Si substrate to terminate said epitaxial growth.
6. The forming method as defined in claim 5, wherein said first
temperature is set within a temperature range of .+-.5.degree. C.
of said Si melting point.
7. The forming method as defined in claim 5, wherein said second
temperature is set within a temperature range between said Si
melting point and .+-.5.degree. C. of said Si melting point.
8. The forming method as defined in claim 1, further comprising the
step of adding into said Si melt at least one element selected from
the group consisting of In, Ga, Sn, Al, Au--Bi and Cu.
9. The forming method as defined in claim 8, wherein a content of
said element is set within 0.01-10 at %.
10. The forming method as defined in claim 8, wherein said liquid
phase epitaxy includes the steps of: supporting said Si melt in a
melt holder formed at a sliding type first member, supporting said
Si substrate in a substrate holder formed at a sliding type second
member, supporting a melt made of said element to be added into
said Si melt in an additive element melt holder formed at said
second member, sliding at least one of said first member and said
second member to contact said Si melt to said additive element melt
and to add said element into said Si melt, sliding at least one of
said first member and said second member when a temperature of said
Si melt approaches to a first temperature around said Si melting
point, to contact said Si melt to said main surface of said Si
substrate and to initiate growth of said Si crystal thin film on
said main surface of said Si substrate through epitaxial growth,
and sliding at least one of said first member and said second
member when said temperature of said Si melt approaches to a second
temperature lower than said Si melting point, to leave away said Si
melt from said main surface of said Si substrate to terminate said
epitaxial growth.
11. The forming method as defined in claim 10, wherein said first
temperature is set within a temperature range between said Si
melting point and -50.degree. C. of said Si melting point.
12. The forming method as defined in claim 10, wherein said second
temperature is set within a temperature range between said Si
melting point and -60.degree. C. of said Si melting point.
13. The forming method as defined in claim 1, wherein said liquid
phase epitaxy includes the steps of: charging said Si melt in a
container, and immersing said Si substrate in said Si melt for a
given period of time to form said Si crystal thin film on said main
surface of said Si substrate through epitaxial growth.
14. The forming method as defined in claim 8, wherein said liquid
phase epitaxy includes the steps of: charging said Si melt in a
container, and immersing said Si substrate in said Si melt for a
given period of time to form said Si crystal thin film on said main
surface of said Si substrate through epitaxial growth.
15. A Si crystal thin film formed by a forming method as defined in
claim 1.
16. A Si crystal thin film formed on a substrate made of
metallurgical Si raw material.
17. The Si crystal thin film as defined in claim 16, comprising at
least one element selected from the group consisting of In, Ga, Sn,
Al, Au--Bi and Cu.
18. The Si crystal thin film as defined in claim 17, wherein a
content of said element is set within
1.times.10.sup.-9-1.times.10.sup.-2 at %.
19. A solar battery comprising a Si crystal thin film as defined in
claim 15.
20. A solar battery comprising a Si crystal thin film as defined in
claim 16.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a method for forming a Si film,
the same Si film, and a solar battery which is made of the same Si
film by means of the same forming method.
[0003] 2. Description of the Telated Art
[0004] In order to diffuse safest and environmental Si solar
batteries on a large scale and on a global scale, it is required to
develop in low cost and in high productivity safest manufacturing
technology of solar battery utilizing global rich sources. As of
now, in both domestically and abroad, such as manufacturing
technology as to complete a solar battery as a device from a Si
melt has been mainly developed by means of casting.
[0005] Since the casting method, however, utilizes a solidifying
method of melt where the temperature gradient of the solid-liquid
interface is increased, it is inherently difficult to develop the
crystal quality of the obtained Si polycrystal. Moreover, since
pure Si raw materials are expensive, the quality of Si raw
materials to be employed is restricted. In this point of view, such
a thin film growing method as to form a Si film on an inorganic
substrate by means of vapor phase epitaxy has been also developed,
in addition to the above-mentioned manufacturing technology.
[0006] With the thin film growing method, however, since epitaxial
growing technique utilizing a crystal substrate is not employed,
but driving force of growth obtained on the large shift from the
equilibrium condition is employed, it is difficult to develop the
crystal quality of the obtained thin film. With the thin film
growing method, in addition, since a substrate made of glass is
employed, the growing temperature can not be enhanced, so that only
micro grains are formed in the thin film, and large grains can not
be formed. Therefore, the efficiency of a solar battery made of the
thin film can not be enhanced.
[0007] In this point of view, as of now, it is eagerly desired to
develop a forming technique of Si crystal thin film of low defect
density and thus, high quality wherein expensive Si raw materials
are not employed and whereby the high efficiency of a solar battery
can be realized.
SUMMERY OF THE INVENTION
[0008] It is an object of the present invention to provide a
forming technique of Si crystal thin film of low defect density and
thus, high quality wherein expensive Si raw materials are not
employed and whereby the high efficiency of a solar battery can be
realized.
[0009] For achieving the above object, this invention relates to a
method for forming a Si thin film, comprising the steps of:
[0010] preparing a Si melt kept at a temperature near Si melting
point,
[0011] preparing a Si substrate made of Si single crystal or Si
polycrystal, and
[0012] contacting the Si melt to a main surface of the Si substrate
to conduct liquid phase epitaxy within a temperature range around
the Si melting point and to form a Si crystal thin film on the main
surface of the Si substrate.
[0013] The inventors had been intensely studied to achieve the
above object, and then, paid an attention to liquid phase epitaxial
(LPE) technique because a crystal thin film of high quality can be
easily formed by the LPE technique under near equilibrium growth
condition. At first, the inventors dissolve Si raw materials in a
solvent made of a melt of low melting point metal to form a
metallic solution with Si therein ,and conduct the LPE technique
utilizing the metallic solution. In this case, however, since the
LPE technique is conducted under lower temperature condition than
the melting point of Si, the intended high crystal quality and
flattened Si crystal thin film can not be formed because of the
large contamination of metallic elements from the metallic solution
and the low growing temperature.
[0014] In this point of view, the inventors made an attempt to
conduct the LPE technique under the condition almost close to the
solid-liquid equilibrium by utilizing the Si melt directly, to form
a low defect density-high quality and flattened Si crystal thin
film. With the LPE technique utilizing the Si melt, even though a
relatively low crystal quality Si substrate is employed, the
crystal quality of the obtained Si thin film is not almost
degraded. Therefore, the Si substrate can be made of a low cost
metallurgical Si raw material, and thus, the manufacturing cost of
the Si crystal thin film can be reduced largely.
[0015] As a result, according to the forming method of the present
invention as mentioned above, the low defect density and thus, high
quality Si crystal thin film can be formed in low cost. In the
fabrication of a solar battery, therefore, if the forming method of
Si crystal thin film according to the present invention is
employed, the conversion efficiency of the solar battery can be
enhanced sufficiently and the manufacturing cost of the solar
battery can be reduced.
[0016] Herein, the wording "metallurgical Si raw material" means
not expensive Si raw material which is not refined sufficiently and
exist in abundance on the globe.
[0017] In a preferred embodiment of the present invention, the LPE
is conducted within a temperature range of .+-.5.degree. C. of Si
melting point. The inherent melting point of Si is 1414.degree. C.,
but fluctuated due to thermal fluctuation, so that the real melting
point is shifted from the inherent melting point. In this case,
since the above-mentioned temperature range is a supercooling
temperature region of Si melt, the Si melt can be maintained in
liquid phase not through solidification. Therefore, the LPE can be
conducted in good condition.
[0018] In another preferred embodiment of the present invention,
the LPE is conducted under the condition that the Si melt is
maintained within a temperature range between the Si melting point
and +5.degree. C. of the Si melting point, and a portion of the Si
substrate is contacted to the Si melt to be melted, and the
temperature of the region of the Si melt in the vicinity of the Si
substrate is maintained lower than the Si melting point. In this
case, the LPE can be conducted under the condition closer to
solid-liquid equilibrium, and thus, the low defect density-high
quality and flattened Si crystal thin film can be obtained.
[0019] In still another preferred embodiment, at least small amount
of one element selected from the group consisting of In, Ga, Sn,
Al, Au--Bi and Cu is added into the Si melt. In this case, the
melting point (solidifying point) of the Si melt can be slightly
reduced, and the melting point zone can be enlarged. Therefore, in
the LPE, the temperature control of the Si melt can be simplified,
and the LPE can be conducted surely.
[0020] In the latter preferred embodiment, the amount of element to
be added can be set within 0.01-10 at %.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] For better understanding of the present invention, reference
is made to the attached drawings, wherein
[0022] FIG. 1 is an explanatory view illustrating one embodiment of
the Si crystal thin film forming method of the present
invention,
[0023] FIG. 2 is also an explanatory view illustrating the
embodiment of the Si crystal thin film forming method of the
present invention,
[0024] FIG. 3 is also an explanatory view illustrating the
embodiment of the Si crystal thin film forming method of the
present invention,
[0025] FIG. 4 is an explanatory view illustrating another
embodiment of the Si crystal thin film forming method of the
present invention,
[0026] FIG. 5 is also an explanatory view illustrating the another
embodiment of the Si crystal thin film forming method of the
present invention,
[0027] FIG. 6 is also an explanatory view illustrating the another
embodiment of the Si crystal thin film forming method of the
present invention, and
[0028] FIG. 7 is an explanatory view illustrating still another
embodiment of the Si crystal thin film forming method of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] This invention will be described in detail hereinafter.
[0030] FIGS. 1-3 relate to one embodiment of the Si crystal thin
film forming method of the present invention wherein a parallel
sliding board is employed. As illustrated in FIG. 1, a first member
11 with a depressed Si melt holder 13 and a second member 12 with a
depressed Si substrate holder 14 are prepared so that the Si melt
holder 13 is opposed to the Si substrate holder 14. The Si melt
holder 13 is capped with a carbon or quartz cap 19. The first
member 11 and the second member 12 are disposed, for example, in a
lateral type furnace (not shown). A rotatable sliding board may be
employed, instead of the parallel sliding board.
[0031] The first member 11 and the second member 12 are connected
to driving shafts 15 and 16, respectively. The driving shafts 15
and 16 are connected to a motor (not shown) so that the first
member 11 and the second member 12 are moved laterally (in
horizontal direction). The driving shafts 15 and 16 may be driven
by man power, instead of the motor. A Si melt X is charged and
supported in the Si melt holder 13, and a Si substrate S is
supported in the Si substrate holder 14.
[0032] The Si substrate S can be made of any kind of Si raw
material, and in the present invention, can be made of meta radical
Si raw material which exist in abundance on the globe. Since the
metallurgical Si raw material is not expensive, the intended Si
crystal thin film can be formed in low cost.
[0033] Under the state illustrated in FIG. 1, the Si melt X is
heated to a given temperature within a temperature range of
1.degree. C..about.100.degree. C. above the Si melting point. The
LPE can be conducted in good condition at a first temperature
wherein the Si melt is not solidified. Concretely, the first
temperature is set within a temperature range of -5.degree.
C..about.+5.degree. C. (.+-..about.5.degree. C.) of Si melting
point.
[0034] Then, the motor (not shown) is driven, and the first member
11 is slid in the left direction with the driving shaft 15 so that
as illustrated in FIG. 2, the Si melt holder 13 is opposed to the
Si substrate holder 14. Then, the Si substrate S is contacted to
the Si melt X so that an intended Si crystal thin film is
epitaxially grown on the Si substrate S.
[0035] Then, after the Si melt X approaches to a second
temperature, the first member 11 is slid in the left direction as
illustrated in FIG. 3 so that the Si melt X is left away from the
Si substrate S to terminate the LPE. The second temperature is not
restricted, but in view of the continuous epitaxial growth,
preferably set to the same temperature as the first temperature or
lower temperature than the first temperature so that the Si melt S
is not solidified. Concretely, the second temperature is preferably
set within a temperature range between -10.degree.
C..about.+5.degree. C. of the Si melting point.
[0036] Through the above-mentioned steps, the intended Si crystal
thin film can be formed on the Si substrate by means of LPE. The
thickness of the Si crystal thin film can be varied by controlling
the contacting period of time between the Si melt X and the Si
substrate S and the temperature of the Si melt X.
[0037] At least one element selected from the group consisting of
In, Ga, Sn, Al, Au--Bi and Cu can be added into the Si melt. The
amount of element to be added can be set within 0.01-10 at %. In
this case, the melting point (solidifying point) of the Si melt can
be reduced, and the melting point zone can be enlarged. Therefore,
in the LPE, the temperature control of the Si melt can be
simplified, and the LPE can be conducted surely.
[0038] When such an additive element as mentioned above is added
into the Si melt X, the additive element is contained in the Si
crystal thin film. If the amount of the additive element to be
added is set within the above-mentioned range, in Ga additive
element with small distribution coefficient, the additive element
content in the Si crystal thin film is set within
1.times.10.sup.-9-1.times.10.sup.-2 at %.
[0039] FIGS. 4-6 relate to another embodiment of the Si crystal
thin film forming method of the present invention. In this
embodiment, too, an intended Si crystal thin film is formed by
utilizing the sliding board. In this embodiment, like or
corresponding members to the ones in the embodiment relating to
FIGS. 4-6 are employed, but the second member 12 includes an
additive element melt holder 17 in addition to the substrate holder
14. Into the additive element holder 17 is charged and supported a
melt of at least one element selected from the group consisting of
In, Ga, Sn, Al, Au--Bi and Cu or Si crystal with a proper amount of
at least one element selected from the group consisting of In, Ga,
Sn, Al, Au--Bi and Cu.
[0040] First of all, as illustrated in FIG. 4, the first member 11
and the second member 12 are prepared so that the Si melt holder 13
is opposed to the additive element melt holder 17. In this case,
the Si melt X is contacted to the additive element melt or the Si
crystal with the additive element Y so that additive elements in
the additive element melt or the Si crystal Y are contained in the
Si melt X through convection and diffusion. The Si melt holder 13
is capped with the carbon or quartz cap 19.
[0041] Then, the Si melt X is heated to a given temperature above
the Si melting point. Since the Si melt X contains the additive
elements, in order to conduct the LPE in good condition, the first
temperature at which the crystal growth starts is set within a
temperature range between the Si melting point and -50.degree. C.
of the Si melting point.
[0042] Then, the motor (not shown) is driven, and the first member
11 is slid in the left direction with the driving shaft 15 so that
as illustrated in FIG. 5, the Si melt holder 13 is opposed to the
Si substrate holder 14. Then, the Si substrate S is contacted to
the Si melt X so that the intended Si crystal thin film is
epitaxially grown on the Si substrate S.
[0043] Then, after the Si melt X approaches to a second
temperature, the first member 11 is slid in the left direction as
illustrated in FIG. 6 so that the Si melt X is left away from the
Si substrate S to terminate the LPE. The second temperature is
preferably set within a temperature range between the Si melting
point and -60.degree. C. of the Si melting point.
[0044] Through the above-mentioned steps, the intended Si crystal
thin film can be formed on the Si substrate by means of LPE. The
thickness of the Si crystal thin film can be also varied by
controlling the contacting period of time between the Si melt X and
the Si substrate S and the temperature of the Si melt X. In this
embodiment, if Ga additive element is employed, the Si crystal thin
film contains the Ga additive element by
1.times.10.sup.-9-1.times.10.sup.-2 at %.
[0045] FIG. 7 relates to still another embodiment of the Si crystal
thin film of the present invention. In FIG. 7, the Si melt X is
charged into a given container 31, and the Si substrate is immersed
into the Si melt X. In this case, the intended Si crystal thin film
can be formed on the Si substrate through epitaxial growth.
[0046] The thickness of the Si crystal thin film can be varied by
controlling the temperature and the immersing period of time of the
Si melt X. Moreover, at least one element selected from the group
consisting of In, Ga, Sn, Al; Au--Bi and Cu can be added into the
Si melt. In this case, too, the melting point (solidifying point)
of the Si melt can be reduced, and the melting point zone can be
enlarged. Therefore, in the LPE, the temperature control of the Si
melt can be simplified, and the LPE can be conducted surely.
EXAMPLE
[0047] In this Example, an intended Si crystal thin film was formed
according to the steps illustrated in FIGS. 1-3. First of all, pure
Si raw material was prepared and heated to 1450.degree. C. to form
the Si melt X. Then, the Si substrate S was made of metallurgical
Si raw material. The Si melt X was kept at 1415.degree. C., and as
illustrated in FIG. 2, the Si substrate S was contacted to the Si
melt X for about 0.5.about.1 minutes during cooling the Si melt at
1.degree. C./min. to form the intended Si crystal thin film on the
Si substrate S through LPE. Then, as illustrated in FIG. 3, the Si
substrate S was left away from the Si melt X to terminate the
LPE.
[0048] The thickness of the Si crystal thin film was 50 .mu.m, and
the lifetime was 5 .mu.s. Then, a solar battery was made of the Si
crystal thin film, and it was turned out that the conversion
efficiency of the solar battery was about 8% on the process of Bell
Communications Research.
[0049] Although the present invention was described in detail with
reference to the above examples, this invention is not limited to
the above disclosure and every kind of variation and modification
may be made without departing from the scope of the present
invention. For example, if a Ge melt is mixed to the Si melt, the
SiGe crystal thin film can be formed, instead of the Si crystal
thin film.
[0050] As mentioned above, according to the present invention can
be provided a forming technique of Si crystal thin film of low
defect density and thus, high quality wherein expensive Si raw
materials are not employed and whereby the high efficiency of a
solar battery can be realized.
* * * * *